Solar energy systems using cadmium telluride (CdTe) photovoltaic (PV) modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner.
Certain factors greatly affect the efficiency of CdTe PV modules in terms of cost and power generation capacity. For example, CdTe is relatively expensive and, thus, efficient utilization (i.e., minimal waste) of the material is a primary cost factor. In addition, the energy conversion efficiency of the module is a factor of certain characteristics of the deposited CdTe film layer. Non-uniformity or defects in the film layer can significantly decrease the output of the module, thereby adding to the cost per unit of power. Also, the ability to process relatively large substrates on an economically sensible commercial scale is a crucial consideration.
CSS (Closed System Sublimation) is a known commercial vapor deposition process for production of CdTe modules. Reference is made, for example, to U.S. Pat. Nos. 6,444,043 and 6,423,565. Within the vapor deposition chamber in a CSS system, the substrate is brought to an opposed position at a relatively small distance (i.e., about 2-3 mm) opposite to a CdTe source. The CdTe material sublimes and deposits onto the surface of the substrate. In the CSS system of U.S. Pat. No. 6,444,043 cited above, the CdTe material is in granular form and is held in a heated receptacle within the vapor deposition chamber. The sublimated material moves through holes in a cover placed over the receptacle and deposits onto the stationary glass surface, which is held at the smallest possible distance (1-2 mm) above the cover frame. It is understood that CSS is a type of diffusive transport deposition (DTD) system, and diffusive transport deposition systems, more broadly, need not necessarily qualify as “close spaced” in nature.
A constant supply of CdTe vapors through the hole plate creates a uniform vapor pressure for deposition onto the substrate. Thus, the deposition rate for the entire CdTe layer can be substantially constant, in an effort to ensure that a substantially uniform thin film layer is formed on the substrate. However, if the initial deposition rate it too fast, voids (i.e., small areas free from CdTe) can be created during the initial deposition. These voids can be exaggerated as the deposition process continues.
Additionally, due to the relatively high temperatures involved in the CSS deposition process, the substrate (e.g., a glass superstrate) can be heated to temperatures that can cause an unregulated curved gradient (e.g., warpage) across the face (i.e., the deposition surface) of the substrate. This unregulated curved gradient can add additional variables into the deposition process. For example, a unregulated curved gradient can induce tensions in the substrate, which can lead to damage in the substrate and/or in the thin film formed thereon. Such curved gradients can be particularly problematic when the substrate has a large surface area and is relatively thin, (e.g., on a glass superstrate of a PV module).
Accordingly, there exists an ongoing need in the industry for an improved vapor deposition apparatus and process for economically feasible large scale production of efficient PV modules, particularly CdTe modules. In particular, a need exists for an improved sublimation plate for use in an economically feasible large scale production of efficient PV modules, particularly CdTe modules, in a CSS process.